The present invention relates to vehicle wheel alignment, and more particularly to compensation schemes for vehicle wheel alignment systems which measure the locations and orientations of the vehicle wheels in a three dimensional coordinate system.
Various systems have been designed to determine vehicle wheel alignment angles. For example, U.S. Pat. No. Re 33,144 to Hunter and January and U.S. Pat. No. 4,319,838 to Grossman and January each describe a wheel alignment system which uses electro-optical transducers to determine the toe alignment angles of a vehicle. FIG. 2 of each of these patents shows six angle transducers carried by support assemblies which are mounted to the vehicle wheels. FIG. 4 of U.S. Pat. No. Re 33,144 and FIG. 9 of U.S. Pat. No. 4,319,838 show the geometry of this arrangement and illustrate the six angles which are directly measured. These patents further describe (see U.S. Pat. No. Re 33,144 col. 7 lines 26-39, and U.S. Pat. No. 4,319,838 col. 8 line 63 to col. 9 line 12) how the toe alignment angles are computed from the angles directly measured by the angle transducers. The disclosures of these patents is hereby incorporated herein by reference.
SAE Publication 850219, titled "Steering Geometry and Caster Measurement", by January, derives and discusses the procedures and methods by which toe and camber alignment transducers are used to determine the caster and steering axis inclination (SAI) of a vehicle. The procedures described therein are the industry standard.
Equipment of this general type and using the apparatus and methods enumerated above has been used world-wide for many years. Such equipment is capable of determining the camber, caster, and pointing or "toe" alignment angles of the wheels relative to one or more appropriate reference axes, and is sufficient to allow proper adjustment of the alignment so as to reduce tire wear and provide for safe handling. It is believed, however, that such equipment could be improved.
German Patent DE 29 48 573 A1, assigned to Siemens A G, describes the use of cameras to determine the locations and orientations of the wheels of a vehicle. On each side of the vehicle, a single camera is moved to multiple positions to view the vehicle wheels. Alternatively, a single fixed camera is used at each side in conjunction with movable mirrors, or multiple cameras are used. The system examines the images thus viewed of the wheels of the vehicle to determine the locations and orientations of the wheels, from which the wheel alignment parameters are determined.
European Patent Application PCT/US93/08333, filed in the name of Jackson and published under the Patent Cooperation Treaty as WO 94/05969 (hereinafter referred to as WO document 94/05969), describes the use of a camera having one or more defined fields of view to view optical targets of known configurations which are mounted to the vehicle wheels. Through the use of sophisticated image recognition methods, the three dimensional coordinates and orientations of the vehicle wheels and their corresponding axes of rotation are determined. The wheel alignment parameters are determined from these coordinates and orientations. The disclosures of U.S. Pat. Nos. 5,675,515, 5,724,128, 5,489,983, 5,488,472, and 5,488,471 are hereby incorporated herein by reference.
There exists a clear need for apparatus and methods which allow a proper determination of the alignment of the vehicle wheels. In particular, there is a clear need for such an apparatus which provides a rapid and accurate compensation of the sensor elements used to measure vehicle alignment angles. For example, proper alignment using video systems is critically dependent upon accurate determination of the positions of the targets in the field of view, which requires accurate initial compensation of the sensor elements/targets.
Prior art compensation makes one large (and sometimes incorrect) assumption--that changes in measurements from one wheel orientation/position to another wheel orientation/position are due to runout and not to changes in alignment. But when a vehicle is rolled from one position to another, lateral forces on the tires can change both camber and toe. Moreover, movement of the steering wheel can change the steer ahead angle. (Even though the vehicle steering wheel is usually leveled and locked, that does not guarantee that the steer ahead angle will not change). In both instances (changes in camber and/or toe, and changes in steer ahead angle) prior art systems interpret these changes as runout which produces an erroneous compensation, and which can result in erroneous alignment measurements.
In one existing video alignment system, a first compensation measurement is taken at a first (forward) position of the vehicle, the vehicle is rolled rearwardly and a second compensation measurement is taken at a second position. Finally, the vehicle is rolled back to the original position and a third compensation measurement is taken. If the measurements are not acceptable, this prior art system gives a warning of wheel wobble, but provides no on-screen information to the user as to the possible (or probable) cause of the wobble. With the prior art system, the user must continue to repeat the procedure until the measurements are accepted, or the user can bypass the compensation altogether. These solutions, however, result in less than desirable accuracy in the subsequent alignment of the vehicle. The prior art can be improved.